The present disclosure relates to an optical recording medium, and particularly to a phase-change optical recording medium including a plurality of laminated recording layers.
A phase-change optical recording medium is common as one of optical recording media capable of recording, reproducing, erasing information by laser irradiation. In the phase-change optical recording medium, information is recorded, reproduced, and erased by using transition between crystal and amorphous phases or between different crystal phases. Examples of commercial products of the phase-change optical recording medium include CD-RW (Compact Disc-Rewritable), DVD-RW (Digital Versatile Disc-Rewritable), DVD-RAM (Digital Versatile Disc-Random Access Memory), and Blu-ray Disc. In recent years, a configuration (two-layer disc) including a plurality of laminated recording layers has been put in practical use in order to increase storage capacity.
The above-described phase-change optical recording medium has the following double-layer disc structure. That is, the optical recording medium has a layer structure in which a first recording layer is formed on a support substrate, a second recording layer is formed on the first recording layer through an intermediate layer transparent to a recording/reproduction wavelength, and a light-transmitting protective layer transparent the recording/reproduction wavelength is formed on the second recording layer. A laser beam used for record/reproduction is incident on an optical recording medium from the light-transmitting protective layer side through an objective lens. The laser beam transmitted through the objective lens is condensed on the first recording layer or the second recording layer to record/reproduce information.
The double-layer disc has a configuration in which the second recording layer is formed as a semi-transmissive recording layer having the light-transmitting performance for recording/reproduction in the first recording layer. The second recording layer includes a dielectric, a metal, and a phase-change recording material which are laminated so as to exhibit the recording/reproducing performance. A typical layer structure includes a dielectric layer, a metal reflective layer, a dielectric layer, a phase-change recording material layer, and a dielectric layer which are laminated in that order from the substrate side. In addition, the second recording layer is designed by imparting a transmittance of about 45% to 55% so that the first recording layer are equal to those of the second recording layer coincide with each other with respect to the recording/reproducing power and reflectance as viewed from an optical disc recording/reproducing apparatus (drive).
As a method for imparting such high light transmittance to the second recording layer (semi-transmissive recording layer), there has been proposed a configuration in which among the layers constituting the second recording layer (semi-transmissive recording layer), the dielectric layer disposed on the substrate side of the metal reflective layer is formed as a transmittance controlling layer by using a material having a predetermined refractive index. Examples of such a material include TiO2, ZrO2, ZnO, Nb2O5, Ta2O5, SiO2, Al2O3, Bi2O3, Ti—N, Zr—N, Nb—N, Ta—N, Si—N, Ge—N, Cr—N, Al—N, Ge—Si—N, Ge—Cr—N, and ZnS. These materials may be used as a mixture (refer to International Publication No. 03/025922 pamphlet).